Hysteresis pacemakers, demand cardiac pacemakers having different pacing and monitoring (standby) rates, are known to the prior art. Such devices are employed to prevent the heartbeat rate from falling below a minimum frequency while pacing the heart at a higher frequency, when needed. In this manner, the complications which may result from a competition between a pacemaker and natural heart activity can be reduced while maintaining a heartbeat rate at a level capable of sustaining life. An example of a prior art demand pacemaker having different pacing and standby rates is disclosed in U.S. Pat. No. 28003 for Electronic Demand Heart Pacemaker With Different Pacing And Standby Rates, reissued May 7, 1974, which is hereby incorporated by reference.
It has been suggested in medical literature that patients having a high risk of sudden cardiovascular death would benefit from the implantation of a standard demand pacemaker. A pacemaker having different standby and pacing rates (i.e., hysteresis pacemaker) may be better suited for these high risk patients since it is less likely to interfere with normal heart rhythms and would only become activated when the patient's intrinsic heart rate becomes sufficiently low to justify pacing. Sudden cardiovascular death may be considered as death occurring within 24 hours of the first symptoms (e.g. heart attack) and usually occurs in less time. With sophisticated diagnostic techniques it is often possible to identify patients who have a high risk of sudden death. For example, modern diagnostic techniques and equipment can identify those patients having a Bifasicular Block (right bundle branch block and left axis deviation accompanied by PR prolongation and/or HV prolongation). The implantation of a pacemaker of the type described above in such a patient would monitor the patient's heart activity and remain inactive so long as the patient's heart continued to beat at a rate above a minimum rate while providing a backup for the likely cessation or reduction of the natural heart activity. The pacemaker would function in the event of a complete cessation of heart activity or the "skipping" of a single heart beat.
Pacemakers of the type described above, when implanted within a "sudden death" likely patient, have the disadvantage of operating at different standby and pacing rates once the necessity of the pacing function is indicated. That is, while the lower standby rate allows the pacemaker to monitor heart activity at a minimum rate to reduce the likelihood of competition with the heart prior to the time that the pacing function becomes necessary, sporadic natural heart activity will cause such pacemakers to periodically revert to the lower standby rate in situations where it may be desirable to monitor the heart at a rate substantially identical to the pacing rate. This, of course, can be overcome by replacing the pacemaker with one having an identity in standby and pacing rates, a typical prior art demand pacemaker, for example. However, the desirability of avoiding such a pacemaker replacement is obvious.